consolidate more policy parsing code

[boneskull]

Find the Venn diagram of parseForPolicy between lavamoat-node, webpack, and core. Move whatever is possible either a) in to core or b) extract it into its own module.

I think (but am likely wrong) this mostly boils down to implementing an importHook function, in parseForPolicy() terms. It may be difficult to tease things apart using this API, however, as these implementations are far from trivial. If we're able, it might be worth attempting to split up or consider something other than a callback-based API.